The present invention relates to a method of determining the rheological characteristics of well drilling fluids according to API (American Petroleum Institute) standards. What is understood to be rheological characteristics is: the rheological curve (or law of variation of the viscosity xcex7 as a function of the shear ({dot over (xcex3)})), the apparent viscosity Va, the plastic viscosity Vp and the yield value YV. The invention allows continuous measurement of these values by means of a device arranged on the mud-return line.
Measurement of the rheological characteristics of well fluids is part of the conventional measurements performed on a drilling site. These measurements, standardized by the API, are not performed on a continuous basis on the mud-return line, but on fluid samples taken in this line or in tanks. Well fluids are characterized in a more or less regular way according to operations. These measurements are carried out, in the best case, by means of a two-speed or six-speed FANN 35 type device. This device is equipped with coaxial cylinders. It determines a shear stress as a function of the shear rate. Other measurements, much more precise but however sporadic, are sometimes performed in the laboratory by means of more sophisticated rotating rheometers.
The object of the present invention is to allow automation of this rheology measurement in order to obtain continuous recording of the characteristics of a drilling fluid. It is thereby expected to obtain more reliable measurements than those currently performed by means of the FANN 35 on fluid samples which are not really representative of the circulating fluid. Monitoring of the drilling operation can be increased and will thus facilitate quasi-immediate detection of anomalies occurring during this operation (water inflow, well fluid degradation, sedimentation, . . . ). This in-line measurement allows to anticipate these technical problems more rapidly. It therefore prevents a disastrous economic impact on the progress of the drilling operation.
In-line rotating rheometers were tested on drilling sites, but they appeared to be inadequate because of the fragility of the rotating parts.
The present invention thus relates to a method for monitoring the evolution of the characteristics of a circulating fluid, for example in a well. According to the invention, the following stages are carried out:
carrying out complex dynamic viscosity measurements with a suitable detector arranged on a line through which said fluid circulates, so as to obtain a law of variation of the complex dynamic viscosity xcex7* as a function of 2xcfx80f, where f is the excitation frequency of the detector,
deducing therefrom the rheogram of the fluid, i.e. a law of variation of the viscosity xcex7 as a function of the shear ({dot over (xcex3)}),
calculating at least one characteristic parameter of the rheology of the fluid from said rheogram,
repeating these stages after a predetermined time interval,
deducing from the evolution of said parameter the evolution of the characteristics of the fluid.
According to the method, the rheogram can be deduced by means of a calibration function depending on the type of detector and on the nature of the fluid.
The calibration function can be obtained from measurements on the initial fluid prior to circulating it.
The measurements can include measurements performed with said detector suited to obtain the complex dynamic viscosity and measurements performed with a FANN type rotating viscometer.
A new calibration function can be calculated upon each fluid change.
Va, Vp and YV can be calculated by means of said rheogram.
Said viscosity variation laws can be modeled according to one of the following models: Ostwald (power law), Hershell-Bulkley, Casson.
The invention also relates to a system for monitoring the evolution of the characteristics of a fluid circulating in a well for example, comprising:
a detector arranged on a line through which said fluid circulates, suited to measure the complex dynamic viscosity,
calculation means for implementing the above method,
means for recording the evolution of the fluid characteristics.